The role of ascorbate in biomembrane energetics.

The mechanism(s) whereby membrane translocations are energized are poorly understood. Our work has focused on transmembrane microsomal and plasma membrane redox constituents as a means to energize membranes via alternative mechanisms complementary to ATP-driven processes. One such component is NADH-ascorbate free radical (mono- or semidehydroascorbate) oxidoreductase. This activity is associated with the trans or exit face of the Golgi apparatus, transport vesicles that move between the Golgi apparatus and the plasma membrane, and with the plasma membrane itself. Various lines of evidence, mostly indirect, link this activity to membrane translocations. Included is an apparent activation of the reductase in membranes when coated with clathrin, a single large polypeptide chain involved in exocytosis and in receptor-mediated and absorptive endocytosis. The results are consistent with a role of the ascorbate free radical as an acceptor for electron transport-mediated transfer of electrons from NADH perhaps to oxygen by coated membranes as a part of a mechanism to drive membrane translocations via generation of a proton gradient or of a membrane potential. Additionally, plasma membrane redox may be important in the regulation of cell growth, but a strict dependence on ascorbate free radical for the latter seems less likely than with internal endomembranes, where redox function may strictly depend upon the restricted pool of regeneratable acceptor that the ascorbate free radical provides.